OKA MERCK VARICELLA VACCINE - A REVIEW OF ITS IMMUNOGENICITY AND PROTECTIVE EFFICACY AGAINST VARICELLA/

Oka/Merck varicella vaccine is a live attenuated vaccine produced from the Oka strain of the herpesvirus varicella tester virus (VZV). The O ka strain of VZV is recognised as having low virulence while eliciting humoral and cell-mediated responses to VZV. In numerous studies, Oka/ Merck varicella vaccine achieved a seroconversion rate of > 95% in hea lthy children and adolescents. In immunocompromised children with leuk aemia in remission, seroconversion was achieved in 80% of vaccinees 12 months after vaccination. Cell-mediated immune responses generally cl osely paralleled humoral immune responses. The vaccine is also immunog enic in healthy adults. In healthy children, a high level of protectio n was conferred by Oka/Merck varicella vaccine, and in the only double -blind placebo-controlled study conducted to date the vaccine was 98% effective in protecting healthy children against varicella during 2 va ricella seasons. In children with leukaemia, Oka/Merck varicella vacci ne was 80% effective in protecting against mild varicella and conferre d 100% protection against moderate and severe varicella. In healthy ad ults, the rate of protection against varicella was reported to be appr oximately 50%. All observed cases of varicella in healthy and immunoco mpromised vaccinees were mild and there were no reports of ay disease- related complications. Interestingly in children and adolescents with leukaemia, the incidence of herpes tester was lower in children who re ceived Oka/Merck varicella vaccine than in similar children who had na tural immunity. Mild varicelliform rash (occasionally accompanied by f ever) has been observed in healthy children, adolescents and adults wh o have received Oka/Merck varicella vaccine. Pain at the injection sit e was the adverse effect most frequently reported in one large study o f the vaccine in healthy children. In children with leukaemia, vaccine -associated rash has been reported in up to 50% of vaccinees. When Oka /Merck varicella vaccine was administered to healthy children in combi nation with measles, mumps and rubella vaccine there was no increase i n the incidence of skin rashes compared with that caused by measles, m umps and rubella vaccine in combination with placebo. Thus, Oka/Merck varicella vaccine is a highly immunogenic vaccine that confers a high level of protection against typical varicella in healthy children and adolescents, and in children and adolescents with leukaemia. While res ults of further long term studies investigating the persistence of pro tective immunity against varicella are awaited with interest, availabl e darn indicate that Oka/Merck varicella vaccine is a major advance in the prevention of this common and highly infectious disease.